This invention relates generally to electrochemical gas sensors and, more particularly, to an improved sensing electrode structure for electrochemical gas sensors. Gas sensors of this type are used to measure the partial pressure of a gas in a mixture of gases. For example, by appropriate selection of components, a gas sensor can indicate the concentration of oxygen in air. Other sensors indicate the concentrations of carbon monoxide, sulfur dioxide, oxides of nitrogen, hydrogen sulfide, and various other gases.
Basically, an electrochemical gas sensor of the type with which the invention is concerned includes a container in which there are disposed an electrolyte, a sensing electrode, and a counter electrode. When a gas to be sensed is introduced into the electrolyte adjacent to the sensing electrode, ions are formed and act as current carriers. A measurable current can then be detected in an external circuit connected to the electrodes.
There are two basic categories of sensors of this general type. One is the galvanic type, in which the counter electrode and electrolyte are selected to provide a measurable current without any external voltage source being necessary. For example, the use of lead or cadmium as a counter electrode, in an alkaline electrolyte, provides a galvanic sensor for the measurement of oxygen concentration. Also, the use of lead dioxide or manganese dioxide as the counter electrode, in an acid electrolyte, provides a galvanic sensor for the measurement of concentrations of carbon monoxide, hydrogen sulfide, and sulfur dioxide. A polarographic sensor is a second detector type, in which the counter electrode is made of material requiring the use of an external voltage source to make the sensor operate. The magnitude of the required external voltage will depend on such factors as the nature of the counter electrode, the acidity (pH) of the electrolyte, and the gas to be measured. The present invention is not limited to either the galvanic or the polarographic type of sensor.
An important consideration in one application of gas sensors is that the readings obtained should be proportional to the partial pressure of the gas to be measured. For example, in monitoring the partial pressure of oxygen in air for health reasons, the reading should reflect changes in total air pressure, even though the oxygen concentrations may not have changed In other words, the sensor should be indicative of the total amount of oxygen available, which is proportional to partial pressure, rather than indicative of the concentration of oxygen by weight or volume.
Responsive to these problems, the inventor received U.S. Pat. No. 4,498,970 issued Feb. 12, 1985, for an Electrochemical Gas Sensor the disclosure of which is incorporated by reference herein.
It has since been discovered that abrupt temperature changes in the sensor's operating environment can affect its detecting abilities. This rapid temperature change induced a thermal expansion of the electrolyte, which raised the pressure of the solution This increased pressure affected the gas solubility characteristics of the electrolyte, distorting the sensor readings. In addition, the increased electrolyte pressure separated the sensing electrode from the porous sheet. As a result, the sensor performance decreased, and gas diffusion time increased. Consequently, users of the sensor were required to thermally re-equilibrate the sensor or, alternatively, raise the test sample and sensor temperature in a step-wise fashion to the desired level. This precluded a rapid and spontaneous testing of the sample.
One proposed solution to this problem is set forth in U.S. Pat. No. 3,767,552 issued Oct. 23, 1973, to J. M. Lauer and provides a galvanic dual membrane system to allow for expansion of the electrolyte solution. In the Lauer sensor, increased pressure within the electrolyte chamber is dissipated by the expansion of a flexible membrane into a juxtaposed expansion chamber. Since this expansion chamber is also open to air distinct from the test sample, gases in the expansion chamber may diffuse into the electrolyte chamber through the flexible membrane and distort the sensor readings.
It will be appreciated from the foregoing that there is still a significant need in the gas sensing field for an electrochemical sensor that overcomes the aforementioned problems of the prior art. In particular, what is needed is a sensor with a relatively low working current, to provide a long useful life, insensitivity to temperature variations, and the ability to respond relatively quickly to changes in partial pressure of the sensed gas. The present invention clearly satisfies all of these requirements.